Simplify code for the gate process plugin

Problem/Motivation

Some of the code in the recently added gate plugin can be simplified.

Proposed resolution

There are three tests like this:

    if (empty($this->configuration['valid_keys']) && !array_key_exists('valid_keys', $this->configuration)) {
      throw new MigrateException('Gate plugin is missing valid_keys configuration.');
    }

Just the second condition is enough:

    if (!array_key_exists('valid_keys', $this->configuration)) {
      throw new MigrateException('Gate plugin is missing valid_keys configuration.');
    }

The line

    $valid_keys = is_array($this->configuration['valid_keys']) ? $this->configuration['valid_keys'] : [$this->configuration['valid_keys']];

can be replaced with

    $valid_keys = (array) $this->configuration['valid_keys'];

Remaining tasks

User interface changes

None

API changes

None

Data model changes

None